1. Field of the Invention
The present invention generally relates to a column repair circuit for repairing a failed cell, and more specifically, to a column repair circuit for performing a column repair operation on memory cell array blocks each having the different number of wordlines.
2. Description of the Prior Art
Generally, in a semiconductor memory device, defective cells are replaced with redundancy memory cells.
When at least one of normal memory cells connected to a column selecting line is defective, a column repair circuit replaces the column selecting line with a redundancy column selecting line. That is, memory cells connected to column selecting lines connected to at least one defective memory cell are replaced with redundancy memory cells connected redundancy column selecting lines.
FIG. 1 is a diagram illustrating a structure of a memory including memory cell array blocks each having the different number of wordlines. Here, 128M memory cell array block is exemplified.
Generally, a DRAM that has a 128M memory cell array block structure is configured to have four 32M banks.
Each 32M bank comprises two groups, and each group comprises 6 memory cell array blocks.
Here, 6 memory cell array blocks included in one group are shown.
In order to increase cell density by reducing the number of bitline sense amplifiers arranged between memory cell array blocks, one memory cell array block is arranged to have 704 or 640 wordlines.
A DRAM comprises a column repair circuit for repairing the failed when a column failure occurs in a memory cell array block,.
However, although one memory cell array block is arranged to have 704 or 640 wordlines, the column repair operation is performed by 512 wordlines.
As shown in FIG. 2a, when a column failure occurs in a second block in the memory cell array block of FIG. 1, the column repair operation is performed only on the third block. Thereafter, as shown in FIG. 2b, when a burn-in test, a reliability test and an application test performed on the rest 512 wordlines and a column failure happens, an additional column repair line is required for the column failure of the same block, which results in degradation of repair efficiency.